1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a method for manufacturing the same, and more particularly, to a surface acoustic wave filter adapted for a high-frequency band and a method for manufacturing the same.
2. Discussion of the Background
A conventional surface acoustic wave filter will be described with reference to FIG. 34. Numeral 341 denotes a piezoelectric substrate, and an input transducer 342 and an output transducer 343 are arranged on the piezoelectric substrate 341. The input transducer 342 and the output transducer 343 are formed of a plurality of combed-type electrode fingers 344 or the like each. The electrode fingers 344 are connected in common by means of bus bars 345, which are connected to electrode pads 346, individually. Grating reflectors 347 are provided outside the input transducer 342 and the output transducer 343.
The electrode fingers 344, bus bars 345, electrode pads 346, and grating reflectors 347 are formed individually as conductive films with predetermined patterns. In many cases, aluminum is used for the conductive films, since it has low resistance and facilitates pattern formation.
In the arrangement described above, electric energy from the input transducer 342 is applied to the piezoelectric substrate 341 and converted into mechanical resonating energy. In contrast with this, mechanical resonating energy from the piezoelectric substrate 341 is converted into electric energy and fetched as a signal from the output transducer 343.
Frequency bands used in the mobile communication field are becoming higher and higher. In surface acoustic wave filters used in mobile communication equipment, therefore, the film thickness of electrodes formed of conductive films and line width are reduced. If the film thickness and line width are reduced, a problem called electro-migration or stress migration arises.
Electro-migration is a failure such that the crystal grains of aluminum or the like, which forms the electrodes, are caused to migrate by current that flows through the electrodes, thus resulting in electrode disconnection. Stress migration is a failure such that the electrodes are vibrated by surface waves or the like, thereby causing mechanical disconnection.
As one of methods for solving the aforesaid problem, there is a method such that a very small quantity of metal such as copper is added to aluminum or some other metallic material that constitutes the electrodes. If about 0.5% by weight of copper is added to aluminum, the time to failure increases several times. If the loading of copper is increased to about 4% by weight, the time to failure extends further.
There is another method in which the conductive films that form the electrodes are stacked in two layers. According to this method, a titanium nitride thin film is formed on a piezoelectric substrate, and an aluminum film is formed thereon, for example. This arrangement improves the problem of electro-migration or stress migration.
According to the method in which copper or some other metal is added, the resistance of the conductive films that form the electrodes is increased by the addition of the metal, so that the insertion loss characteristic of a surface acoustic wave filter, one of the basic characteristics thereof, worsens. For this reason, the loading of copper cannot be increased. In the conventional surface acoustic wave filter, therefore, the electrodes sometimes may be destroyed by input power of only about 1 W despite the addition of copper.
In the case of the method in which the conductive films are stacked in two layers, moreover, upper and lower layers are formed of aluminum and titanium nitride, respectively, for example. In this case, a gas, e.g., boron trichloride or chlorine gas, used in etching aluminum in the upper layer cannot be used to etch titanium nitride in the lower layer. Therefore, forming predetermined electrode patterns by etching the conductive films requires dedicated safety equipment, such as a gas line or purifier, for titanium nitride etching. Accordingly, the investment in this equipment and maintenance expenses increase, thus raising the manufacturing cost of products.
In the surface acoustic wave filter, as mentioned before, input and output electrical signals are processed by utilizing mechanical vibration or so-called surface vibration of that region of the piezoelectric substrate in which input and output electrodes are formed and conversion into voltages or other electrical signals. In this case, the film thickness of the conductive films is settled depending on the required electrical properties.
The properties of surface acoustic waves that propagate through the piezoelectric substrate vary depending on the presence of the conductive films on the piezoelectric substrate. They also vary depending on the mass and thickness of the conductive films on the piezoelectric substrate. The thickness of the conductive films has a value corresponding to a desired frequency characteristic, which is substantially inversely proportional to the frequency and proportional to the wavelength.
In the case where LiTaO3 is used for the piezoelectric substrate, for example, the film thickness of aluminum that forms the conductive films should be about 150 nm in order to obtain the frequency characteristic in the GHz zone. In order to obtain the frequency characteristic of 400 MHz or thereabout, the film thickness of aluminum should be about 550 nm.
In the surface acoustic wave filter described above, the thickness of the electrode fingers formed on the piezoelectric substrate is settled depending on the required frequency characteristic. However, the electrode pads and bus bars should have a great film thickness in consideration of their purposes. For example, metal wires are connected to the electrode pads. Steady bonding with the metal wires requires the thickness of 300 nm or more. The bus bars are utilized for signal conduction. In this case, a lower electric resistance and greater film thickness are preferred in order to reduce the insertion loss.
In the conventional surface acoustic wave filter, however, the individual electrodes, such as the electrode fingers, electrode pads, and bus bars, are all formed in the same process. Therefore, the respective thickness of the conductive films of the electrode pads and bus bars are defined by the film thickness of the electrode fingers that is settle depending on the required frequency characteristic. If the film thickness of the electrode fingers is reduced with the enhancement of the frequency characteristic or the like, the electrode pads and bus bars are also thinned. In consequence, the bonding with the metal wires becomes unsteady or the insertion loss becomes greater, so that the reliability lowers, and the electrical properties worsen.
If the width of the electrode fingers becomes finer with the enhancement of the frequency characteristic, moreover, static electricity may accumulate between the paired electrode fingers that constitute the input and output electrodes, thereby destroying the input and output electrodes, in some cases. As a method for preventing such destruction, there is a method in which the bus bars to which the paired electrode fingers are connected in common are connected by means of a metallized portion that has a resistance of 300 to several kxcexa9 or more. In this case, however, the width of the metallized portion is as small as about 10 xcexcm. In a manufacturing process or the like, therefore, the metallized portion may possibly be disconnected.
Normally, a chip that constitutes a surface acoustic wave filter element is stored in a ceramic package. In this case, electrode pads of the surface acoustic wave filter element and an external circuit are connected by wire-bonding using metallic wires of gold or aluminum. In the case of this method, a side wall portion or the like of the package is provided with connecting pads for connection with the metallic wires. Thus, the connecting pads require an installation space, thereby adding to the size of the package and making it difficult to miniaturize the surface acoustic wave filter.
Accordingly, the flip-chip bonding technique is used to reduce the size of the package. This is a method such that bumps of gold or the like formed on the electrode pads of the surface acoustic wave filter element are used for connection with the external circuit. According to this method, the connecting pads are unnecessary, so that the package can be reduced in size.
When using the flip-chip bonding technique, compared to the wire-bonding, however, the electrode pads are subjected to a great force when the electrode pads and the external circuit are connected. If the film thickness of the electrode pads is reduced by the enhanced frequency characteristic of the surface acoustic wave filter, therefore, an external force may reach the piezoelectric substrate, thereby damaging the piezoelectric substrate, in some cases. This may possibly be avoided by increasing the film thickness of the electrode pads with use of the lift-off method. If the film thickness of the electrode pads is increased by the lift-off method, however, a problem arises such that the conductive films that form the electrode pads and conductive films stacked in layers by the lift-off method should be separated.
With the enhancement of the frequency characteristic, as mentioned before, the conventional surface acoustic wave filter has come to be expected to cope with electro-migration and stress migration. Further, the mechanical strength of the electrode pads and the electrical properties of the bus bars must be improved. Moreover, a measure is needed to counter the electrostatic destruction of the input and output electrodes.
The object of the present invention is to provide a surface acoustic wave filter capable of solving the above problems and adapted for the enhancement of the frequency characteristic and a method for manufacturing the same.